Since it has been passed about forty years from finding or discovery of laser, the laser technology is utilized in wide variety of technical fields. Particularly, in the technical field for measuring physical action, very high accuracy measurement can be made by making use of high coherent characteristic of a laser beam. For example, in the field where very small mechanical vibration is measured, a laser beam is irradiated onto an object to be measured and a reflected light is monitored, whereby high accuracy analysis of very small vibration is carried out.
On the other hand, for the purpose to monitor voice, a system having a microphone is still utilized from the past several ten years up to the present. However, in such a conventional system having a microphone, there is a problem that it is difficult to monitor voice from remote place. For this reason, in order to monitor voice in a room from the outside of the room, it is necessary to install a microphone within the room or in the vicinity of the room. However, in special or particular circumstances such as criminal investigation, etc., there are many instances where it is difficult to install a microphone within the room to be monitored or in the vicinity thereof, and there is no margin in point of time. In such a case, hitherto, it was inevitably required to employ a remote monitoring method in which a microphone having uni-directivity is installed at a remote place from the room and voice wave propagated through the space is collected by the microphone.
In view of the above, an object of this invention is to provide a voice monitoring system using laser beam which can monitor voice within a room from a remote place.